The ability to manipulate the bone marrow output of various blood cells has become an important tool in the management of several diseases. Some of the best new therapies for hematological malignancies are based on the development of compounds that push leukemic cells to differentiate into lineages to which they are committed prior to the transforming event. One such example is the case of acute promyelocytic leukemia. Upon treatment of patients with Arsenic Trioxide, the malignant cells are pushed along the myelomonocytic pathway leading to remission of those tumors. Another example lies in promotion of successful engraftment of transplanted bone marrow stem cells (long term reconstituting hematopoietic stem cells, or lt-HSC) in irradiated individuals. The appearance of differentiated blood cells can be accelerated by the systemic administration of cytokines that are known to specifically induce red blood cell development (erythropoietin, or Epo), or myeloid cell development (granulocyte-macrophage colony-stimulating factor, or GM-CSF). Finally, harvesting of lt-HSC from donors has been greatly simplified by the process of “mobilization” wherein these cells are induced to move from the bone marrow sites where they normally reside into peripheral blood by systemic administration of a cytokine called G-CSF. Stem cells can then by harvested from peripheral blood obviating the painful and elaborate collection of bone marrow biopsies. All of these processes rely on the ability to program and control the biological behavior of lt-HSC.
Accordingly, bone marrow (stem cell) transplantation is an invaluable therapeutic tool for hematologic and immune reconstitution of individuals who have undergone radiation and/or chemotherapy (e.g. cancer patients, or have been exposed to high-level radiation), and is also a critical modality for treatment of immune deficiency and hematological malignancies. In addition, bone marrow transplantation would be a highly useful therapy to combat the negative effects of aging on the immune system, as well as on other cells and tissues. It is estimated that stem cell transplantation could benefit more than 35,000 children and adults per year.
The operative principle behind bone marrow transplantation is replacement of radiation sensitive lt-HSC that give rise to all blood cell types. Recent studies indicate that bone marrow transplantation may have value in the treatment of heart disease. Although the basis of this affect is unknown, it, and other findings, raise the possibility that hematopoietic stem cells (lt-HSC) may be reprogrammed to give rise to other tissues. If this is true, lt-HSC may have much broader utility and provide an alternative to controversial embryonic stem cell therapy.
The major obstacles confronting clinical application of bone marrow transplantation lie first in identification of an appropriately histocompatible marrow donor. This is usually accomplished using registries that have enrolled more than 6 million potential donors. The selected donor must undergo a grueling ordeal of induced mobilization stem cell into the blood followed by 4-5 days of leukapheresis to isolate rare lt-HSC. Transplantation of these cells must be followed by careful monitoring and treatment of the recipient to minimize graft versus host reactions caused by passenger lymphocytes.
Elucidation of the molecular basis of the impairment in hematopoietic lineage development has been complicated historically by the low frequency of relevant cell populations, which prevents biochemical analysis of signaling and downstream responses. In fact, this has been a major limiting factor in all studies of hematopoiesis. In addition, the limited availability of long-term hematopoietic stem cells (LT-HSCs) has also been a major obstacle in the treatment of many types of cancer as well as several kinds of immune deficiencies in humans. To the best of the present inventors' knowledge, there are currently no available cell lines that arose spontaneously that resemble lt-HSCs and can differentiate into normal lineages in vitro, or that can reconstitute lethally irradiated mice or sub-lethally irradiated humans, nor have any methods been described to deliberately generate such cell lines. Moreover, there are currently no viable technologies to continuously expand lt-HSCs, such that these cells need to be obtained from a donor every time they are needed.
There is also a dire need for additional modalities to treat hematological malignancies and immune deficiency, and novel cytokines to increase the output of transplanted lt-HSC. In addition, an appropriate platform for target identification and drug discovery does not currently exist. The missing elements are cell lines that represent different developmental stages in hematopoietic lineages. Optimally, such cells should retain the ability to undergo further differentiation in a specific lineage. Such cell lines are essential for identification of gene products, and thus new drugable targets, involved in regulation of cell development, proliferation and survival. In addition, such cell lines are essential for the screening of small molecule and shRNA libraries for loss-of function studies, as well as cDNA libraries for gain of function studies, in search of novel drugs.
Barriers to current drug discovery in this area include: (a) isolation of a sufficient number of cells from a particular developmental stage; (b) propagation of the cells in vitro for a sufficient length of time; and (c) ability to use conditional oncogenes to screen for drugs that could affect leukemic cells, and not normal HSCs or progenitors.
Therefore, there is a great need in the art for a method to generate lt-HSC cell lines that can be expanded extensively, frozen, and used again whenever they are required, in the absence of subsequent harvests from the donor.